Effects of Cu, N, Mn and Al Additions on Structures and Properties of Cr-Ni Stainless Steels

Abstract

An investigation was made into the effects of Cu, N, Mn and Al additions on structures of Cr-Ni stainless steels and of the effects of the structures on mechanical properties and corrosion resistance.
Experimental results obtained were as follows:
i) Specimens containing Cu were hardened by precipitation of secondary ferrite and those containing N were hardened mainly by precipitation of Cr₂N and by transformation of austenite to ferrite or martensite. Specimens containing Mn and those containing Al were hardened by formation of σ phase from σ ferrite and by precipitation of Al₃Ni respectively.
ii) The grain boundary reaction which was characterized by formation of lamellar nodules occurred during aging at 750.. with specimens containing higher N content. Granular chromium carbide precipitated within grains of σ ferrite in aging with specimens containing higher Al content. σ phase was formed more rapidly from σ ferrite in aging at 750.. with specimens containing higher Mn content.
iii) The nickel equivalence of Cu and N in their ability to extend the austenite region at high temperature was about 05 and 20 respectively. The Cr equivalence of Al as a ferrite former was about 4. Addition of Mn to the steels decreased the amount of σ ferrite, but on increasing the Mn to above 4% it slightly increased the amount of σ ferrite.
iv) Tensile strength of specimens containing Cu and that of specimens containing Mn depended mainly upon the structural conditions. Solution-hardening by N was very large but effect of structural conditions on tensile strength was larger than that of solution-hardening at test temperature below 500... Tensile strength of specimens containing Al depended mainly upon the amount of AIN.
v) Increase of creep-rupture strength with increasing Cu and N contents was due to both solution-hardening by Cu and N and decrement of the amount of σ ferrite with increasing Cu and N content.
Creep-rupture strength of specimens containing Mn and that of specimens containing Al depended only upon the amount of σ ferrite.
vi) With specimens containing N, the corrosion resistance to HNO₃ reached minimum at about 0.2% of N and that to H₂SO₄ reached maximum at the same content of N.
With specimens containing the other elements, the relation between corrosion resistance to H₂SO₄ and the content of alloying elements was the same as that between creep rupture strength and the content of alloying elements.